1. Field of the Invention
The present invention relates to an ESD protection device, and particularly to an ESD protection device useful when used in a high-speed transmission system or combined with a common mode filter.
2. Description of the Related Art
Miniaturization and sophistication of electronic apparatus have rapidly been advancing in recent years. Considerable increase in transmission speed (frequency higher than 1 GHz) and decrease in drive voltages have also been achieved, as represented by USB 2.0, S-ATA2, HDMI, and other high-speed transmission systems. In contrast, withstand voltages of electronic components used in electronic apparatus decrease as the electronic apparatus are reduced in size and the drive voltages are reduced in amplitude. An important technical challenge in this case is to protect electronic components from overvoltages represented by electrostatic pulses produced when the human body comes into contact with a terminal of an electronic apparatus.
In related art, to protect electronic components from electrostatic pulses of this type, a multilayer varistor is typically provided between an electrostatically vulnerable line and a ground. However, large capacitance of a multilayer varistor, when used in a high-speed transmission system, causes reduction in signal quality.
A low-capacitance antistatic component that has been proposed has an electrostatic protection material filled between electrodes facing each other. For example, Patent Document 1 discloses an electric circuit protection device (antistatic component) with a voltage variable polymer material placed between electrodes. The electric circuit protection device is formed by applying a conductive particle-containing polymer material into a gap between the electrodes by using stencil printing and solidifying the material in a heat treatment. Patent Document 2 discloses an electrostatic protection material paste obtained by kneading metal particles, each of which has a passive layer formed thereon to enhance a static electricity reduction effect, a silicone-based resin, and an organic solvent. Patent Document 2 also discloses an antistatic component with an electrostatic protection material layer between a pair of electrodes. The antistatic component is formed by applying the electrostatic protection material paste into the space between the electrodes facing each other by using screen printing and drying the resultant structure.
Patent Document 3 discloses a ceramic paste containing a metal oxide, a resin component, and a solvent component. Patent Document 3 also discloses an electric circuit protection device (antistatic component) with a voltage-dependent resistive layer primarily made of zinc oxide formed therein. The electric circuit protection device is formed by burying the ceramic paste between electrode paste films by using screen printing and then sintering the resultant structure at a high temperature.    [Patent Document 1] National Publication of International Patent Application No. 2002-538601    [Patent Document 2] Japanese Patent Laid-Open No. 2007-265713    [Patent Document 3] Japanese Patent Laid-Open No. 2004-006594
However, the antistatic components described in Patent Documents 1 to 3 are characterized in that an organic-inorganic composite film that functions as nonlinear resistance is formed by applying an organic-inorganic composite material containing a resin component and metal particles or a metal oxide and then drying the material. A problem, therefore, is that the thus formed organic-inorganic composite film has poor heat resistance and the characteristics of the film tend to change with temperature, humidity, and other environmental conditions. Further, since the metal particles or the metal oxide undergoes aggregation, segregation, or other phenomena during the preparation process of the organic-inorganic composite material and during the formation process of the organic-inorganic composite film, the overvoltage protection effect of the thus formed film tends to be non-uniform, resulting in poor reliability.
Further, in the antistatic components described in Patent Documents 1 to 3, since the organic-inorganic composite films are formed by using stencil printing, screen printing, or any other suitable method to apply the organic-inorganic composite film materials and drying the resultant structures or treating them in other processes, productivity and cost effectiveness are poor. Moreover, the thickness of the organic-inorganic composite film formed by any of the methods described above is typically at least several tens of micrometers. It is therefore difficult to further reduce the thickness of the organic-inorganic composite film without compromising the productivity and cost effectiveness.
The present invention has been made in view of the above circumstances. An object of the present invention is to provide an ESD protection device that has low capacitance, excellent discharge characteristics, and improved heat resistance and weather resistance. Another object of the present invention is to provide an ESD protection device that has a further reduced thickness and excels in productivity and cost effectiveness.